Definition
A font of special characters such as mathematical symbols, dingbats, or other non-alphabetic sorts not found in a standard text font. Pi fonts are used to supplement regular typefaces with specialised characters. The name derives from the traditional printer's term "pi" for mixed or disordered type.
Source
— Robert Bringhurst — p. 328:
A font of special characters such as mathematical symbols, dingbats, or other non-alphabetic sorts.